There are several techniques of the prior art that have been used to increase the speed of scanning computer files by antivirus software.
For example, the software product known as Norton AntiVirus (NAV) manufactured by Symantec Corporation runs continuously in the background of a processor. If a file is modified, it is automatically rescanned by NAV. The NAV server-based antivirus software keeps a cache of files that have been scanned and certified clean (virus-free) since the last reboot of the server. If such a file is later accessed by the user, NAV does not rescan the file, since NAV knows that the file is already clean. Such a technique works well for servers, because servers are rarely rebooted and the same files are used over and over again. However, on desktop (client) computers that are reset frequently, such a cache cannot be maintained for long periods, because desktop computers are rebooted frequently. Furthermore, desktop computers typically contain a relatively low amount of memory.
In a second technique of the prior art, desktop based antivirus programs, such as IBM's AntiVirus, store hash data for each program on the hard drive to speed up scanning operations. Once a file is scanned, a hash value (or simply "hash") of the contents of the file is stored in a database. The hash value is a contraction of the file contents created by a hash function, which may or may not be specifically tailored to the type of the file. Hash functions are described in Schneier, Bruce, Applied Cryptography 2d ed. (John Wiley & Sons, Inc.), Chapter 18, pp. 429-460, U.S.A.
A hash function is a many-to-one function, i.e., more than one file configuration can have the same hash value, although this is highly unlikely. In this prior art technique, during subsequent scans of the file, the hash of the file is first computed by the antivirus software, and if the computed hash matches the hash stored in the database, the file is certified clean by the antivirus software without the necessity for a rescan. This is possible because a match shows, with a high degree of certainty, that the file has not been modified. This technique eliminates the need for costly CPU-intensive rescans of the file.
Currently, the prior art techniques either take a hash of the entire file or specifically tailor their hash to critical areas of the file based upon the internal file format. If these critical areas change, there is a possibility of viral infection. If the areas do not change, the likelihood of viral infection is reduced and the file is not rescanned.
A second company that has a technology for hashing files on a desktop computer and rescanning them only if the hash values have changed is Sophos Ltd. of the United Kingdom.